The following abbreviations are utilized herein:
3GPP third generation partnership project
ACK acknowledgement
A/N ACK/NACK
ARQ automatic repeat-request
AT allocation table
BS base station
CAZAC constant amplitude zero autocorrelation
CDM code division multiplexing
CQI channel quality indicator
CRC cyclic redundancy check
DFT discrete Fourier transform
DL downlink (Node B to UE)
DTX discontinuous transmission
E-UTRAN evolved UMTS terrestrial radio access network
eNodeB enhanced Node-B
FDD frequency division duplex
HARQ hybrid automatic repeat-request
I/Q inverse/quadrature
L1 layer 1 (physical layer, PHY)
LTE long term evolution of UTRAN
MIMO multiple input/multiple output
NACK negative acknowledgement
PDCCH physical downlink control channel
PDSCH physical downlink shared channel
PRB physical resource block
PUSCH physical uplink shared channel
Node B base station
OFDM orthogonal frequency division multiplexed
OFDMA orthogonal frequency division multiple access
PMI pre-coding matrix indicator
PRB physical resource block
PUSCH physical uplink shared channel
RLC radio link control
RS reference signal
SINR signal to interference-plus-noise ratio
SNR signal to noise ratio
TDD time division duplex
TDM time division multiplexed
TTI transmission time interval
UE user equipment, such as a mobile station or mobile terminal
UL uplink (UE to Node B)
UMTS universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
A proposed communication system known as E-UTRAN or LTE is currently under discussion within the 3GPP. The E-UTRAN or LTE system is a packet-based system that operates under strict control of the BS (Node B). The usage of physical UL/DL resources is signaled from the eNodeB to the UE, typically on a TTI per TTI time scale. The signaling is realized by use of UL and DL ATs (also termed PDCCHs). The UL and DL ATs indicate to the UE which physical resources are assigned for UL and DL data transmissions, respectively. When data transmission occurs over a wireless medium, there is a risk of error when receiving and detecting the data.
From the point of view of the UL, there are a number of potential signaling error events related to DL resource allocation:
(1) The reception of DL allocation grant fails (i.e. only the DL allocation was sent).
(2) Both UL and DL allocations fail.
(3) The DL allocation grant fails but the UL allocation grant does not.
A working assumption in the 3GPP is that UL and DL allocation tables are separately encoded (e.g., error (3)). Error (2) may occur, for example, in the situation where the UL and DL ATs are jointly coded. Similarly, error (2) can occur when uplink and downlink allocation tables are separately coded, and both fail simultaneously. The error rate related to the resource allocation signaling is assumed to be on the order of 1%-5%.
Note that the ACK/NACK caused by the DL AT and the UL data allocation caused by UL AT will probably be associated to different TTIs. This is due to the fact that ACK/NACK signaling cannot be transmitted until the corresponding DL data packet has been decoded. This is in contrast to the UL data since the UL data can be transmitted immediately once the UL AT has been correctly received.
It is also possible for the UE to decode the DL control channel but the CRC check fails (i.e., there was a recourse allocation for the given UE but it cannot be utilized).
In Layer 1 specifications of 3GPP LTE standardization (e.g., TS 36.211, 36.212 and 36.213) are discussed multiple HARQ-ACK transmission in PUSCH for TDD mode of LTE system. There are some differences between TDD and FDD modes regarding to control signalling in general. In the FDD mode, each DL sub-frame has a dedicated UL sub-frame to be used to transmit DL related L1/L2 control signals such as ACK/NACK. In the TDD mode, a single UL sub-frame needs to support signalling of L1/L2 control signals from multiple DL sub-frames. The number of DL sub-frames associated with a single UL sub-frame depends on the DL-UL ratio, which is configured by the eNodeB (e.g., broadcasted in system information). Clearly the more dynamic TDD mode is more difficult.
Two different approaches have been discussed in 3GPP regarding to the ACK/NACK signalling in TDD mode. One is termed ACK/NACK bundling, in which ACK/NACK feedback related to multiple DL sub-frames is compressed into a single ACK/NACK feedback transmitted via a single ACK/NACK resource. The other is termed Multi-ACK/NACK (also known as ACK/NACK multiplexing), in which each DL sub-frame is considered as a separate HARQ process. A separate ACK/NACK feedback is transmitted for each (granted) DL sub-frame. Specification work related to ACK/NACK bundling is almost completed. The multi-ACK/NACK scheme is currently approaching the final agreement pending from the exact HARQ-ACK/NACK feedback information in LTE Rel. 8 specifications.
The 3GPP specification at the time of this invention does not support explicit DTX detection for ACK/NACK over the PUSCH in LTE TDD, or to the extent one may consider that it does, it does so with high signaling overhead. LTE as currently understood may be considered to support explicit DTX detection for ACK/NACK over PUSCH because each HARQ process could use a 3-state feedback. This represents a high overhead for explicit DTX detection because it would require K bits instead of N bits for the 3-state feedback, where K=ceil(log 2(3^N). Taking specific examples to illustrate the overhead increase, for N=2 then K=4; for N=3 then K=5; for N=4 then K=7; etc. Though this does provide the full information/capability for the eNodeB to identify the explicit DTX state, it requires from 66.7% to 100% more signaling overhead (either 2 to 3 additional bits) to have such explicit DTX detection. On the other hand, if e.g. the last consecutive subframe(s) are missed by the UE, there is a risk that the eNodeB may wrongly decode the HARQ-ACK/NACK feedback from all detected DL assignments due to the wrong encoding of HARQ, and so therefore the UE may have to always encode the HARQ-ACK 3-state feedback based on the “worse” case, i.e. N always equal to the number of associated DL subframes per PUSCH regardless of how many DL subframes are actually scheduled. Thus to support explicit DTX signaling, LTE currently would have each DL HARQ process use a 3-state feedback (ACK, NACK and DTX), and the number of required signaling bits would be K=ceil(log 2(3^N)).
Consider an example. In the case of 4DL subframes being associated with 1UL subframe, if the eNodeB scheduled 3 DL subframes to one UE and the UE missed the last DL AT (assignment), instead of encoding the HARQ-ACK from 3 subframes the UE will encode the HARQ-ACK from 2 subframes, while the eNodeB is trying to decode the HARQ-ACK assuming it was encoded from 3 subframes thus can not correctly decode anything because of the eNodeB's wrong assumption about the encoding. This comes from the reason that in current LTE TDD Rel8 specifications the UE can not detect the missed last consecutive DL assignments, i.e. the UE assumes that there is not a DL assignment sent if those missed DL assignments are from the last consecutive ones. But the UE can detect the missed DL assignments if not being the last consecutive ones, thus the DTX state can be generated for those missed DL assignment. To solve such mis-matched encoding and decoding, the UE has to encode the HARQ-ACK of 4 DL subframes always with 3 state feedback per DL subframe. This means that the encoded feedback is always 7-bits (from 4 DL subframes with 3-state from each), regardless of the actual number of DL assignment, e.g. with 1, 2, 3 or 4 DL assignment the number of required feedback bits are 2, 4, 5 or 7 for 3-state feedback per DL assignment or 1, 2, 3 or 4 bits for 2-state feedback per DL assignment. Apparently, the signaling overhead is huge, up to 600% (in the case of 1 DL assignment was sent by the eNodeB but the UE encodes a 7-bits feedback). Certain teachings herein according to the second aspect of the invention detail how to improve upon this high overhead.